A liquid crystal display device has advantages such as thinness, lightweight, low power consumption and so on, and is widely used in various technical fields such as computers, word processors, television sets, mobile phones, handheld terminal devices and so on. In addition, so-called touch panels of such liquid crystal display devices having a mechanics of manipulation by touch on their screen are spreading rapidly. The touch panels are widely used in mobile phones like smart phones, tablet computers, handheld terminal devices, automated teller machines, automatic vending machines, personal digital assistances, copying machines, facsimiles, game machines, guiding devices set up in facilities such as museums and department stores, car navigation systems, multifunctional terminals set up in convenience stores or monitoring devices of railroad vehicles.
The touch panel generally has a transparent electroconductive laminate comprising a transparent substrate on which a transparent electrically conductive layer is formed. Indium tin oxide is generally used to form a transparent electroconductive layer.
A PET film or a polycarbonate film is commonly used as a substrate film of a transparent electroconductive laminate thanks to its high transparency and reasonable prices. A transparent hard coat layer can be preferably applied onto such a substrate film with a view to improving scratch resistance and durability. On the other hand, the application of a transparent hard coat layer onto a substrate causes a problem of the appearance of interference fringes. The appearance of interference fringes deteriorates visibility.
The appearance of interference fringes is due to a difference of refractive indexes between a transparent substrate firm and a transparent hard coat layer, and quite minute variation of the thickness of the transparent hard coat layer. The appearance of interference fringes can notionally be vanished by through suppression of variation of the thickness. However such a measure is unrealistic in current technologies and is extremely difficult in practice.
Other means to prevent the appearance of interference fringes include a means to minimize a refractive index difference between a transparent substrate firm and a transparent hard coat layer. For example, JP 2009-265590 A (Patent Document 1) discloses that the addition of a high refractive index metal oxide like zirconia, titanium dioxide, indium tin oxide (ITO), antimony tin oxide (ATO), zinc oxide (ZnO), tin oxide and zinc antimonate as a high refractive index material into a transparent hard coat layer can control a refractive index of the transparent hard coat layer. However, the addition of a high refractive index material such as the metal oxide into a transparent hard coat layer may deteriorate stretchability and flex resistance of the hard coat layer.
JP 2002-241527 A (Patent Document 2) describes a film having a transparent hard coat layer with excellent visibility. Patent Document 2 further describes a high refractive index layer. In Patent Document 2, a high refractive index layer is formed by vapor deposition or sputtering of a metal oxide such as ZnO, TiO2, CeO2, SnO2, ZrO2, ITO, or by dispersing fine particles of the metal oxide in a binder resin (paragraph 0007 and so on).
JP 2003-055607 A (Patent Document 3) describes a coating material causing less visible interference fringes on a transparent substrate. The coating material has a controlled attenuation factor due to compounding of fine particles and/or a pigment. The fine particles include those of various metals, metal oxides, metal nitrides and metal carbides. The pigment includes an azo pigment, a phthalocyanine pigment, an anthraquinone pigment, a quinacridone pigment, a dioxane pigment, a quinacridone pigment (paragraph 0021 and so on).
Patent Documents 1-3 describe a means of the prevention of appearance of interference fringes and the improvement of visibility. However, Patent Documents 1-3 disclose the technical features different from those of the present invention in that Patent Documents 1-3 use the high refractive index materials such as metal oxides.
JP 2007-292883 A (Patent Document 4) describes an optical laminate having a high refractive index hard coat layer containing a bromoresin (claim 1 and so on). Patent Document 4 discloses that the optical laminate has improved optical properties, light stability and hardness. The present invention has different technical features from the Patent Document 4 in that the present invention uses no bromoresin to increase the refractive index of the hard coat layer.
JP 2008-239673 A (Patent Document 5) describes a transparent crosslinked film which is obtained by curing and crosslinking a composition comprising a vinyl ester composition, a polyfunctional acrylate and a (meth)acrylate having a fluorene backbone (claim 1). The transparent crosslinked film is characterized in that it has excellent surface hardness and no interference fringe. In Patent Document 5, experimental tests were performed under a condition of a film thickness of 50 μm. However, such a film thickness is an extremely large thickness compared to a film thickness commonly used in the field of hard coat layers. Such an extremely large film thickness provides disadvantages in a cost. A suppression effect of interference fringes obtained with the composition taught by Patent Document 5 does not reach a level required in the technical field at present.